On the gradients of ACR oxygen at intermediate heliocentric distances: Ulysses/Soho results

Abstract

During 1997 and up to the present time, the heliocentric position of the Ulysses spacecraft has changed only slowly in distance and latitude, being close to the equator at a distance of ~5 AU. This period provides an excellent opportunity to observe the evolution of the Anomalous Cosmic Ray (ACR) spectrum at a location intermediate between 1 AU and the outer heliosphere. We present ACR observations in the energy range ~3 to 30 MeV/nucleon made using the COSPIN/LET instrument on board Ulysses and the ERNE experiment on SOHO at 1 AU. Based on these observations, which cover a latitude range of ~0° to 20° with respect to the heliographic equator, we find a radial gradient G_r of 18.0+/-2.4 %/AU and a latitudinal gradient G_λ of 0.6+/-0.1 %/deg for ACR oxygen at 10 MeV/n, with corresponding values of 25.0+/-3.0 %/AU and 0.9+/-0.1 %/deg at 5.5 MeV/n.